The present invention relates to a rotary punching device, and more particularly to a rotary punching device with improved punching quality.
Conventional rotary punching device which is incorporated in a copy machine and others to punch a circular hole at the edge of a copy sheet is well known. As shown in FIG. 7, in such a rotary punching device, a punch holder 3 with a mounted punch 2 is mounted on one rotary shaft 1 of two rotary shafts provided in parallel whereas a die holder 6 with a dice 5 is mounted on the other rotary shaft 4. A release hole 4a of the die 5 and a paper discharging hole 6a of the die holder 6 and the hole 3a of the rotary shaft 4 are aligned in a straight line.
Gear wheels (not shown) are engaged in the two rotary shafts 1 and 4, respectively so as to be meshed with each other. When either one of rotary shafts is driven by a motor, punch 2 and die 5 are synchronously rotated at a constant speed. Thus, the punch 2 and the die 5 repeat engagement and disengagement.
As shown in FIG. 24(a), the tip surface of the cylindrical punch 2 is formed in a flat shape. As shown in FIG. 7, the punch 2 is inserted into a punch attachment hole of the punch holder 3 engaged on the rotary shaft 1 and secured there by a fastening screw (not shown). The die 5 is inserted into a die attachment hold of a die holder 6 engaged on the rotary shaft 4 and secured there by a fastening screw (not shown).
The rotary punching device is configured so that the sheet feeding speed of a sheet feeding mechanism for feeding a sheet P between the two rotary shafts 1 and 4 is equal to the circumferential speed of the punch 2 and die 5. Thus, the punch 2 and dice 5 rotate in synchronism with the fed sheet to punch a hole in the sheet P. The punched sheet piece is externally discharged from the release hole 4a of the die 5 through the hole 3a of the rotary shaft 4 and the sheet-piece discharging hole 6a of the die holder 6.
Assuming that the circumferential speed of the tip of the punch 2 and die 5 is V.sub.1, the linear speed V.sub.1x of the punch 2 and die 5 in a sheet feeding direction is V.sub.1x =-V.sub.1 cos .theta. which varies at a period of 360.degree.. As shown in FIG. 22, when the circumferential speed V.sub.1 of the punch 2 and die 5 is set to be equal to a sheet feeding speed V.sub.2, the linear speed V.sub.1x of the punch 2 and die 5 coincides with the feeding speed V.sub.2 of the sheet P only at the rotation angle of 180.degree. of the punch 2. Before and after the angle of 180.degree., the linear speed V.sub.1x of the punch 2 and die 5 is lowered with respect to the feeding speed V.sub.2.
For this reason, because of changes in the linear speed V.sub.1x of the punch 2 and the die 5 in an engagement range (150.degree.-210.degree.) between the punch 2 and die 5, various inconveniences such as deformation of the punching hole and rupture of the edge thereof will occur.
In order to prevent rupture of the punching hole, a rotary punching device has been proposed in which the circumferential speed V.sub.1 of the punch 2 and die 5 is set to be slightly higher than the sheet feeding speed V.sub.2 so that as shown in FIG. 23, V.sub.1x =V.sub.2 at an engagement starting point (150.degree.) and an engagement ending point (210.degree.). In this case, however, V.sub.1x &gt;V.sub.2 between the above two points and hence the longitudinal form of the punching hole is a short ellipse.
The conventional rotary punching device has problems such as breakage and deformation of the punching hole due to a change in the relative speed between the punch and die driven at a constant rotation speed and a sheet fed at a constant linear speed. This gives rise to a technical problem to be solved in order to improve the quality of the punch hole.
In the rotary punching device, if a sheet-piece is discharged at a waiting position where the die stops rotation, it will be discharged at substantially the same position. But the sheet piece does not necessarily always drop at the same time due to various causes, such as accumulation of sheet pieces in the release hole of the die and influence by static electricity, and is frequently discharged during the rotation of the die. Thus, a large amount of sheet pieces will be dispersed.
This gives rise to a technical problem of controlling the discharging direction of punched sheet pieces so as to be always constant, thereby making cleaning easy.
FIG. 25 is a graph showing the punching load of the above rotary punching device. As seen from the figure, it exhibits a concave-shape load curve with peaks at the starting and ending points of cutting and particularly the maximum peak at the ending point.
This is because, as shown in FIG. 26, the cutting length of the punching hole per a unit of rotating angle .theta. increases in the second half of a cutting stroke to reach the maximum at the end point of cutting. For this reason, the driving motor for the punch and die requires large torque, and the rotary shafts 1 and 4 require high warping rigidity, thus hindering the miniaturization of the driving mechanism.
This gives rise to a technical problem of reducing the peak of the punching torque to relax the load of the device.
Furthermore, the sheet feeding device may provide a variation in the sheet feeding speed because of changes in the diameter due to abrasion of a sheet feeding roller and in the friction coefficient on the surface of the roller. In this case, a difference occurs between the sheet feeding speed and the linear speed of the punch and die, thus leading to poor punching. For example, the shape of the punching hole may be deformed and the edge of the punching hole may be broken. Further, the linear speed of the punch arranged upstream may exceed the sheet feeding speed of the sheet feeding downstream on a sheet feeding path so that the intermediate portion of the sheet floats from the sheet guide. The sheet may flutter to produce abnormal sound.
This gives rise to a technical problem of preventing the variance between the sheet feeding speed and the linear speed of a punch and a die from being generated, thereby stabilizing punching quality. The present invention intends to solve the above problem.